1. Field of the Invention
This invention relates to an autofocus adapter that can be applied to focus detection in autofocus control of an imaging lens.
2. Description of Related Art
An autofocus system in a home-use video camera, etc., generally is based on a contrast system. This contrast system integrates high-frequency components of video signals in one range (focus area) of the video signals obtained from an imaging device (luminance signals) to find a focus evaluation value and automatically makes focus adjustment so that the focus evaluation value becomes the maximum. Accordingly, the best focus for allowing the sharpness (contrast) of an image captured by the imaging device to become the maximum is obtained.
However, the contrast system is a so-called hill-climbing system for finding the best focus while moving a focusing lens and thus has a disadvantage of a reaction rate to focusing. Then, to eliminate the disadvantage of the contrast system, a method of detecting the focus state of an imaging lens using a plurality of imaging devices placed at positions different in the optical path length is proposed. (Refer to JP-A-2002-296492, JP-A-2002-365517, JP-A-2003-248164 and JP-A-2003-270517.) In the detection method, an imaging device for focus state detection is disposed at three positions of a conjugate position relative to a usual imaging device for image capturing and positions at an equal distance preceding and following the position, focus evaluation values are found from the video signals obtained from the imaging devices for focus state detection, and a comparison is made between the focus evaluation values in the greater-than, equal-to, less-than relation, whereby the focus state on the light reception face of the usual imaging device for image formation is detected. An imaging device for focus state detection is not placed at a conjugate position relative to an imaging device for image formation and is disposed only at two positions at an equal distance preceding and following the position, so that the focus state can also be detected. The method of detecting the focus state of the imaging lens using a plurality of imaging devices has the advantage that the reaction rate to focusing is also high because not only whether or not the state is an in-focus state can be determined, but also whether the focus state is ahead or behind the in-focus position can be determined.
By the way, some zoom lenses for a broadcast camera, etc., contain a relay lens system into which an extender optical system can be inserted. JP-A-2002-365517 and JP-A-2003-270517 propose each a system of inserting a light split unit into a relay lens system in a imaging lens for splitting subject light and introducing the split subject light for focus state detection into each imaging device for focus state detection. On the other hand, JP-A-2002-296492 and JP-A-2003-248164 propose each a system of splitting subject light behind a imaging lens rather than splitting a light flux in a imaging lens and introducing the subject light for focus state detection. This is an adapter system placed detachably between the imaging lens and the camera main body and can be attached to already existing general imaging lenses as an autofocus adapter.
However, in the system using the relay lens system in the imaging lens as described in JP-A-2002-365517 and JP-A-2003-270517, if the optical system for focus state detection is incorporated, it becomes impossible to use an extender optical system. It cannot be applied to an imaging lens including no relay lens system; this is a problem. On the other hand, each of the systems described in JP-A-2002-296492 and JP-A-2003-248164 is an adapter system placed detachably between the imaging lens and the camera main body and can be attached to already existing general imaging lenses. In JP-A-2003-248164, however, the adapter optical system contains the relay optical system and an image is once formed in the adapter optical system and then the image formation light is captured in the imaging device of the camera main body by the relay optical system as a relay system. Thus, the overall length at the attachment time becomes long and the system lacks compactness. JP-A-2002-296492 does not describe a specific lens configuration in the adapter optical system. Therefore, development of specific lenses suited to the adapter optical system is demanded.